1. Field of the Invention
The present invention relates to a mobile communication system for avoiding collision among RSs (Reference Signals).
2. Description of the Related Art
In view of limitations of transmission power in a user terminal, an SC-FDMA (Single Carrier Frequency Division Multiple Access) scheme is proposed for an uplink radio access scheme in E-UTRA (Evolved UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access) standardized by 3GPP (3rd Generation Partnership Project), in order to reduce PAPR (Peak to Average Power Ratio) and to improve the efficiency of the transmitting amplifier (see “3GPP TR 25.813” V1.0.1 (2006-06), 3GPP Organizational Partners (ARIB, ATIS, CCSA, ETSI, TTA, TTC), for example). FIG. 1 shows an example of uplink frequency allocation in an SC-FDMA radio access scheme. In FIG. 1, signals for four user terminals UE1-UE4 are allocated on the frequency axis. It should be noted that a frequency band used by each user terminal UE1-UE4 changes according to a channel condition or the like.
Regarding an uplink subframe configuration in the SC-FDMA radio access scheme, it is proposed that SC-FDMA symbols for RSs and SC-FDMA symbols for data signals be multiplexed according to TDM (Time Division Multiplexing). FIG. 2 shows an example of an uplink subframe format in the SC-FDMA radio access scheme. In FIG. 2, fourteen SC-FDMA symbols (symbols #0-#13) are placed with each gap CP (Cyclic Prefix) placed therebetween. RSs are transmitted from each user terminal at the timings of the symbols #0, #3, and #10.
Regarding an RS sequence used for channel estimation during synchronous detection and/or for reception quality measurement, it is proposed that a CAZAC (Constant Amplitude Zero Auto Correlation) sequence be used in order to improve the accuracy of channel estimation due to good autocorrelation properties. The use of different CAZAC codes among cells can reduce interference from other cells. In addition, the use of the same CAZAC sequence with a cyclic shift in user terminals which use the same frequency band in the same cell can orthogonalize signals transmitted from user terminals.
In addition, the SC-FDMA radio access scheme can increase the coverage by narrowing the transmission bandwidth and increasing signal power density of the transmission bandwidth. In other words, the SC-FDMA radio access scheme can increase the range of transmission signals.